1. Field of the Invention
The present invention relates to an optical deflecting-scanning apparatus used in such image forming apparatus as laser beam printers or digital copiers.
2. Related Background Art
The optical deflecting-scanning apparatus used in such image forming apparatus as laser beam printers or digital copiers is constructed to reflect a light beam such as a laser beam by a rotary polygon mirror and deflect the beam to scan with high-speed rotation of the rotary polygon mirror. The scanning light thus obtained is focused on a photosensitive member on a rotating drum to form an electrostatic latent image thereon. Then the electrostatic latent image on the photosensitive member is developed into a toner image by a developing device, this image is transferred onto a recording medium such as a recording sheet, then the sheet is sent to a fixing device, and the toner on the recording medium is heated to be fixed by the fixing device, thereby achieving printing (print).
With increase in speed of the digital copiers etc., there are recently developed machines with the rotating speed of the rotary polygon mirror being over 30,000 rpm.
FIGS. 1A and 1B illustrate the main portion of an optical deflecting-scanning apparatus according to a conventional example, wherein the apparatus has a bearing 101 supported on an optical box (not illustrated) a motor 102 comprised of a rotor rotatably supported by the bearing 101 and a stator opposed thereto, and a rotary polygon mirror 103 integrated with the rotor by an elastic press mechanism or the like.
When the stator of the motor 102 is excited by driving current supplied from a driving circuit on a motor board, the rotor rotates the rotary polygon mirror 103 at high speed so as to deflect the light beam irradiating the rotary polygon mirror 103 to scan.
The rotary polygon mirror 103 is a polygonal member of a polygonal prism shape having a plurality of reflective surfaces, and the upper part of the rotary polygon mirror 103 and the motor is covered by a cover member 104. As the rotary polygon mirror 103 rotates at high speed, the air around it flows to cause wind noise or the like. Therefore, the apparatus is designed so as to prevent the wind noise or the like from leaking to the outside of the apparatus, by covering the periphery of the rotary polygon mirror 103 by the cover member 104.
The peripheral wall of the cover member 104 is provided with a window 104a opening so as not to intercept the light beam L.sub.0 incident into and emergent from the rotary polygon mirror 103. The window 104a is an aperture formed by removing a part in an arcuate shape from the peripheral wall of the cover member 104, and a dustproof glass member 105 is attached to the window 104a.
With a conventional technology described above, however, the pressure gradient occurs in the air in the cover member with rotation of the rotary polygon mirror, so as to increase vibration sound etc. of the motor, and there is an unsolved issue that the noise cannot be suppressed sufficiently by simply covering the periphery of the rotary polygon mirror by the cover member.
In general, the noise appearing with high-speed rotation of the rotary polygon mirror results from superposition of various types of noise, which can be roughly classified under three types. The first type is the wind noise (air sound) produced when the rotary polygon mirror causes the air in the cover member to flow with rotation thereof, and leaking from the optical box; the second type is the vibration sound caused in such a way that rotation of the rotary polygon mirror creates a pressure gradient in the air in the cover member and that the stator (coil) of the motor vibrates with receiving reaction force when the rotary polygon mirror is rotated at high speed against the load; the third type is great noise caused by resonance or forced excitation or the like when such vibration of the stator propagates to the cover member and the optical box.
The cover member of the conventional example is designed to focus on only the first wind noise and simply covers the periphery of the rotary polygon mirror so as to prevent the wind noise from leaking to the outside. As long as the rotating speed of the rotary polygon mirror is relatively low, the wind noise itself is small and is not so significant even with a little leakage thereof from the cover member. The wind noise leaking from the cover member, however, increases with increase in the speed of the optical deflecting-scanning apparatus, and the second and third vibration sound is added to this, thereby resulting in heavy noise.
FIG. 2 illustrates the air flow created in the cover member with rotation of the rotary polygon mirror, from which it is observed that the density of the air flow is small near the window of the cover member and as a consequence, the pressure of the air is locally decreased to create the pressure gradient.